¹Fisheries & Oceans Canada, 867 Lakeshore Road, Burlington, Ontario, Canada, L7R 4A6
²Plankton Canada, Burlington, Ontario, Canada

*Corresponding author: munawarm@dfo-mpo.gc.ca

Summary

   Microbial loop (bacteria, autotrophic picoplankton, and heterotrophic nanoflagellates) phytoplankton, size fractionated primary productivity, and zooplankton from the Canadian waters of Lake Erie was studied at 15 stations on a biweekly basin from April - November 1998 using standard methodologies. Annual mean phytoplankton biomass suggested that the west basin was meso-eutrophic (3.0 g m^-3); the central basin was mesotrophic (1.7 g m^-3) and east basin was also mesotrophic (1.9 g m^-3). The size structure (ESD) indicated a prevalence of nanoplankton (2-20 µm). Chlorophyta was increasingly important in the community structure of all the 3 basins. Species diversity was high across the lake ranging from 183 – 207 identified species. Size fractionated primary productivity was highest in the west basin and nanoplankton was dominant in all three basins. P/B quotients were highest for picoplankton (<2 µm) throughout the lake. Zooplankton biomass was also highest in the west basin, with a decreasing gradient across the lake. Cladocerans dominated the biomass in the west and central basins however Calanoids were most prevalent in the east. Comparisons of trophic interactions revealed some interesting and contrasting predator prey relationships amongst the three basins. Phytoplankton as a primary food resource was plentiful across the lake with sufficient edible forms amenable for grazing including Diatomeae, Chlorophyta and various phytoflagellates. However the composition of the predators was significantly different among basins. In the west basin, herbivorous zooplankton and dreissenid veligers were dominant whereas herbivorous zooplankton and carnivorous zooplankton were prevalent in the central basin. On the other hand, the east basin was overwhelmingly dominated by carnivorous zooplankton. The trophic interactions studied during 1998 present an interesting and complex picture of three lakes contained within a single lake, each with characteristic food web interactions. The intensive sampling, as done in the Lake Erie Biomonitoring (LEB) study of 1998, could serve as a model for future plankton investigations and be supplemented with laboratory grazing experiments in order to fully comprehend the food web dynamics of a lake which is changing rapidly.